1. Field of the Invention
The invention is related to a light emitting device having a light emitting element and a phosphor, which can be used for various light sources such as a signal light, lighting, a display, an indicator. Especially, the present invention provides the light emitting device which can emit a white light by using the phosphor which is excited by the emission spectrum from the light emitting element and capable of emitting a visible light.
2. Description of the Related Art
Various light emitting diodes and laser diodes are developed as a semiconductor light emitting device today. Such a semiconductor light emitting device has begun to substitute for an electric bulb and a cold cathode tube as various light sources, such as a display, a back light and an indicator. These light sources use the strong points of a low-voltage drive, small sizing, lightweight, thin shape, high reliability with a long-life, low electrical power consumption.
Especially, the light emitting element using a nitride semiconductor has been developed as a light emitting element which can emit a light in a range from the ultraviolet region to the short wavelength side of visible light. The blue or green LED of more than 10 candela has an active (light emitting) layer of quantum well structure made of nitride semiconductor (for example, mixed crystal of InGaN) and has been developed and commercialized.
A display of mixed-color including white is realized by combination of the light from such a LED chip 1 and the light from the phosphor which is excited by the emitted light. For example, such devices are mentioned in the JP,5-152609,A, JP,9-153645,A, JP,10-242513,A, etc.
More concretely, the light emitting element emits an ultraviolet light or a blue light which has a relatively short wavelength in visible light. It is the light emitting diode which excites a phosphor in order to emit the visible light having a wavelength longer than the wavelength of the light emitting element. When a part of light from a light emitting element is used as transmitted light, there are advantages such that the structure can be simplified and the output can be easily improved.
On the other hand, when a light emitting element which emits an ultraviolet light is used, a white light is emitted by combining the phosphors which can emit RGB (red, green, blue) lights. In this configuration, since the lights emitted from the phosphors are basically used only, a color adjustment can be performed comparatively easily. Especially, when a wavelength of the ultraviolet region is used, since the variations, such as wavelength of a semiconductor light emitting element, are canceled and the chromaticity is determined by only luminescence colors of the phosphors, the productivity is improved as compared with the case where the semiconductor light emitting element which emits a visible light is used.
However, to utilize the light emitting device with the advantages of semiconductor light emitting elements as a light source such as a lighting, current light emitting devices are not enough, so improvement in the brightness and improvement in the productivity are required.
Especially, the phosphors, which can be excited by the ultraviolet light or the light of short wavelength in the visible range, and which can emit a red light with sufficient brightness as compared with other blue, green, etc., is not known. For this reason, the luminescent color made from the three primary colors only from phosphors has little red component, so such a luminescent color is felt as a somber color since the red component is not reproduced. On the other hand, if mixed rate of a red phosphor is raised to improve the color rendering property, relative luminance will fall.
If the excitation spectrum curve of the phosphor with respect to the luminescent spectrum of the light emitting element is varied significantly, although the luminescent spectrum emitted from the light emitting element is positioned in a region of ultraviolet range or visible range of which visual sensitivity is very small, the variation in a color tone may arise between the light emitting devices, because the slight variation of the excitation spectrum from the light emitting element generate variation of the brightness of the phosphor.
Furthermore, when the excitation light spectral band width of a phosphor is narrow, the reliability of the light emitting device will be spoiled by the ultraviolet or near-ultraviolet light which is not absorbed by the phosphor among the light emitted from the light emitting element. If organic materials are used for the package housing the light emitting element, sealing member for protecting the light emitting element, the binder member for the phosphor, the output power is depredated and luminescent color is shifted by coloring or degradation of the organic materials caused by the light which is not absorbed by the phosphor.
Moreover, in order to improve productivity, it is necessary to realize various desired colors by adjusting composition of one phosphor.